Liquid-liquid extraction has long been used to transfer one or more components in a first liquid to a second liquid which is substantially immiscible with the first liquid. Typically, the liquid to be treated is contacted with a suitable substantially immiscible second liquid having a different specific gravity from the first, and which preferentially extracts one or more components from the first liquid. After the extraction step, the two liquids are allowed to separate by gravity. The component extracted by liquid-liquid extraction may be liquid, solid, or in ionic form.
In one example of liquid-liquid extraction, a component carried in inorganic solvent (such as water) is removed (extracted) from an aqueous phase by contacting the aqueous phase with an organic liquid (such as kerosene) phase, followed by phase separation. Similarly, acidic or basic components of an organic solution are removed by contacting the organic solution with an alkaline or acidic aqueous solution, as appropriate.
In another use of liquid-liquid extraction, an aqueous solution containing an ionic species, say, copper ions, is contacted with a liquid "ion exchange material", which forms all or part of an organic liquid immiscible with the aqueous solution. When the two liquids contact each other, the ionic species leaves the aqueous phase to combine with the ion exchange material (ion exchanger) in the organic phase to form a compound soluble in the organic phase, but insoluble in the aqueous phase. An example of a liquid ion exchanger soluble in a liquid hydrocarbon vehicle, such as kerosene, is a hydroxy oxime ion exchanger, which is used to extract copper from acidic or basic aqueous solutions containing copper ions. A typical hydroxy oxime ion exchanger is sold under the trademark "LIX 84N" by Henkel Corporation, Tucson, Ariz.
In a typical liquid-liquid extraction process, the two immiscible liquids of different specific gravities are brought into contact to cause the mass transfer of one or more components from one liquid to the other. Thereafter, the two liquids are permitted to separate by gravity. The mixing and subsequent separation is called an extraction stage. In each such stage, one liquid is dispersed in the other, and the resulting dispersion is allowed to settle out to separate the two liquids. The equipment used for each extraction stage is usually called a mixer-settler, or an extractor.
Many liquid-liquid extraction operations use a plurality of mixer-settler units connected in series to provide for countercurrent flow as the aqueous phase moves through the mixer-settlers in one direction, while the organic phase moves in the opposite direction. For example, a liquid-liquid extraction plant can include one or more mixer-settler units that make up an "extraction section" in which a component is transferred from one liquid (the feed) into a second liquid (the extractant), as described above. The plant also usually includes one or more mixer-settler units in series with the extraction section to provide a "stripping section", where each mixer-settler acts as a "stripper" to remove extracted component from the extractant. In the mixer-settler units of the stripping section, the organic liquid extractant, i.e., the second liquid loaded with the component transferred from the first liquid, contacts an immiscible stripper liquid which removes or strips the transferred component from the extractant to the stripper liquid. This regenerates the extractant, which is recovered from the stripping section and recycled back to the extraction section for removing more component from additional first liquid carrying the component.
That part of the first liquid from which component has been removed (the "raffinate") can either be discarded or reused. For example, U.S. Pat. No. 4,083,758 (1978) to Hamby et al discloses process and apparatus for recovering metallic copper from chloride-containing spent etching solutions, and for regenerating the etching solution for reuse. In the Hamby et al patent, spent chloride-containing ammoniacal aqueous etching solution from making printed circuit boards is treated with an ion exchanger in an organic liquid to remove copper ions so the spent etching solution can be regenerated and reused to dissolve additional copper from printed circuit boards. The copper removed by the liquid ion exchanger is stripped from the exchanger liquid with a sulfuric acid stripper solution, which regenerates the ion exchanger so it can be reused. Copper is removed from the stripper solution by electrowinning so the sulfuric acid can be recycled for recovering additional copper.
According to the Hamby et al patent, the organic ion exchanger also removes chloride ions with the copper ions. The chloride ions present a hazard during the electrowinning of the copper because chlorine gas is released. Therefore, the chloride ions are removed by washing the pregnant organic liquid with a weak aqueous acidic solution. This washing step creates a waste solution containing chloride ions, which presents a waste disposal problem.
The carryover of unwanted chloride ions with the copper ions, as disclosed in the Hamby et al patent, is an example of "cross-contamination", which limits the use of liquid-liquid extraction, especially as waste disposal requirements become increasingly stringent and expensive.
Cross-contamination and slow phase separation by the action of gravity imposes a severe restraint on liquid-liquid extraction. The rate of transfer of a component from one liquid to another depends on the effective surface area of contact between the two liquids. Ideally, for a maximum transfer rate of component from one liquid to another, the two liquids should be thoroughly homogenized, i.e., agitated into a suspension of tiny droplets of one liquid in the other as a continuous phase. This would result in a high rate of mass transfer of a component from one liquid to the other, but the liquids would be difficult to separate by gravity, and would require large retention time and settling tanks to avoid unacceptable cross-contamination. Consequently, the two liquids are normally agitated together in a relatively gentle fashion to avoid over-dispersion or emulsification of one liquid into the other so that separation can be obtained by gravity settling within a reasonable amount of time and space. Even then, a small amount of each liquid is entrained in the other, resulting in cross-contamination which adversely affects product quality, throughput, and operating costs.
A variety of mixer-settler units have been designed to improve liquid-liquid contact and to minimize cross-contamination in liquid-liquid ion exchange processes. For example, U.S. Pat. No. 3,989,467 to Paige discloses equipment which includes a mixing tank where two liquids are gently mixed to form a dispersion which then flows into a shallow settling tank with a large horizontal cross-sectional area located adjacent the mixing tank. As the dispersion flows horizontally across the settling tank, three layers form. The upper and lower layers are formed by the coalesced phases as the two liquids separate from the dispersion. The middle layer, which is usually only a few inches deep, is a dispersion of one of the liquids in the other. The separated phases are removed from the end of the settling tank remote from the mixing tank by suitably located weirs which retain the dispersion layer in the tank. The large horizontal area of conventional gravity settlers, such as the unit just described, requires an undesirably large inventory of organic liquids in the settler. In addition, such horizontal units take up an inordinate amount of floor space. Attempts to reduce the cross-sectional area of such settling tanks for a given flow rate results in a disproportionate increase in the depth of the dispersion layer and, accordingly, can result in flooding of the settler with dispersion. This results in carryover of one of the liquids in the other (cross-contamination) and causes inefficiencies in operation.
U.S. Pat. No. 4,221,658 to Hardwick (1980) addresses the problem of undesirably large organic inventories in settling tanks of large horizontal cross-sectional area by providing a generally vertical mixer-settler unit where mixing and settling are carried out in the same vessel. My U.S. Pat. Nos. 4,595,571 (1986) and 4,657,401 (1987) disclose an improved liquid-liquid extractor which minimizes required floor space and reduces the problem of carryover or cross-contamination arising from small amounts of one liquid phase being entrained in the other liquid. As indicated above in the discussion of the Hamby et al patent, cross-contamination can be reduced by using special wash stages to remove contaminating materials. However, the additional wash stages are not always entirely effective. Moreover, the effluent from such wash columns presents an environmental problem of disposing of the effluent from the wash columns.